Protective device for induction furnace



, Aug. 15, 1933. F. T. cHEsNuT I PROTECTIVE DEVICE FOR INDUCTION FURNACES Filed July 22. 1931 FIG.1

Patented Aug. l15, 1933 1,922,029 y PROTECTIVE DEVICE Foa INDUCTION ACE Frank Theodore Chesnut, Yardley, Pa., assigner to Ajax Electrothermlc Corporation,

Ajax

Park, N. J., a Corporation of New Jersey Applicaton'luly 22, 14931. Serial No. 552,388

- ,somma thereby preventing dangerous or damaging arcs f the application from forming between coil turns or between the Icoil and charge in the furnace.

'I'he main purpose of my invention is to provide a method of detecting short circuits formed by voltage breakdowns between the coil and charge in an induction electric furnace and to protect apparatus land operators by discontinuing the supply of current to .the furnace before damage can be done.y

v A further purpose of my invention is to prevent anexplosion or violent expulsion of a molten or solid charge from a furnace due to water being trapped beneath the charge and' converted to steamby the intense heat.

A further purpose is to supply a method of making a permanent electrical connection to a' charge being heated or to a bath of molten metal in the-furnace.

A further' purpose is to supply a visible means of informing a furnace operator that the electri.

cal connections to the charge are good. Y

A further purpose is to sound an alarm when aA breakdown has occurred between coil and charge and to cause, in addition.l a visible signal to be made.

A fm'ther purpose is to substitute a shield of conducting material for the second contact point in the protective device to be used, in place of the coll, to anticipate metal leaks.

A further purpose is to stop the flow `of cooling water to the inductor coil and apply-compressed air to'blow out what water is left in the coilto further protect against an explosion due to breakdown.

tions and in the claims. v

I have preferred to illustrate my invention by six figures'. Y

Fig. 1 shows a cross section of a furnace embodying my invention.

Fig. 2 is similar to Fig. 1 except that a conducting shield is used as one side of the protective circuit instead of the furnace coil. A

Fig. 3 is a fragmentary view showing eral form of shield used in Fig. 2. Fig. 4 is a cross section of a furnace showing of -my invention to non-fluid charges.

the gen- Fig. 5 shows a method or bringing in the cen-.-

tact leads to the charge from the topof the furnace through an insulating or protecting sleeve.

Further purposes will appear in thespeciflca-A Fig. 6 shows a method of cutting o vthe flow of cooling water and clearing the inductor coil by means of compressed air in case of trouble.

In the drawing, similar numerals indicate like parts.

Protective devices of the type or for the reasons described have not found favor in thecom` mon practice of operating induction furnaces because the methods thus-far proposed have been uncertain of action, hard to install or by their installation have caused more serious trouble. ln additionto this, since breakdowns have been few and of comparative negligible violence, it has not been thought necessary or economical to in- In practically all commercial induction fur-v naces of the coreless type, the inductor coil is of hollow tubing, water cooled. The turns of this coil are energized by a comparatively high voltage, and they are separated from the charge by only a thin refractory wall. When the refractory wall becomes hot, or when it cracks and metal vfromthe charge approaches it, the eifec- 'tive coil insulation is impaired, and there is danger of an arc forming between coil turns and the charge proper. This arc may take the form of a double breakdown where the voltage jumps from one turn to the charge and then back 'to another turn, or it may occur between adjacent turns if metal has'leaked out -to make contact between these An arc'cannot form, however, by metal leaking out to, or even touching, one single turn. l

When an arc does form, it can sometimes be detected by its noise and the switch can be opened manually in time to prevent damage. If the is not opened immediately, a section isv Several' protective devices are now known. One

coil proper. Metal leaks from the furnace are supposed to open or short circuit these two wires is composed of a two wire winding inside of the before reaching the coil. Another method comprises burying a shield, and a metal leak from the charge on making contact with both shield and coil actuates safety devices. Still another method comprises two buried shields. These are connected to a protective circuit and metal from the charge makes contact with both to operate the device.

' Burying a coil or shield which protects all of the lateral sides of a coil is a diiiicult operation. The space is not great, and there is always danger of leaving spaces blank, or causing a weakened refractory insulation by its addition.

In my invention, I have resorted to simple and positive protective means. In no way `have I impaired insulation. My protection is anticipatory for molten metal charges, operating before an arc has had opportunity to form, and operating at the first contact of charge and coil. It is positive of action and shows its connections to be in good order until a break actually does occur. When a break occurs, it is instantaneous in its protection and warning.

Referring to the figures, I shall now describe what I claim as my invention.

In Fig. 1 I have shown furnace leads l and 71 feeding an inductor coil 1, suitably surrounding a molten metallic charge 3, which is being maintained in a sintered refractory lining 2. The gure shows a sintered lining, but my invention or protective device is equally applicable where a Crucible is used to hold the melt or charge.

yWiresfl and 5 are buried in the refractory, or projected through the crucible before packing, and are made of a material which will not contaminate the charge being melted. For iron and steel melts I propose an iron wire, for instance, of a size 12 to 18 B and S gauge. It should be understood that I d o not limit myself to any particular connector, desiring only to establish metallic contact with the charge. One of these wires 4, is connected to the ground 12, and to one side of a suitable alternating or direct current control source 8, 81. The other wire 5, is connected to the other side of this control source through the lamp 13. Thus it may be seen that the burning of the lamp 13, insures that contact wires 4 and 5 are connected to the charge. 'I'hese wires will become hot and will melt off, but will always maintain contact with the-'charge To doubly insure this contact, several wires may be used and they may enter the charge from either bottom or sides of the refractory wall. If desired, they may enter from the top of the furnace and may be led down through the refractory through a quartz or insulating tube 19, as shown in Fig. 5. In this case, it becomes of little consequence if the refractory opens up around the wire, as the metal `cannot leak through to the outside.

Lead 81 from the control source is further connected to the solenoid 11, and to a mid tap on the inductor coil 1. This solenoid when energized attracts the armature 10, opening contacts 9 in the main circuit and closing contacts 14.` The solenoid 1l, can only operate when the control circuit is closed by the metal pool or charge coming in contact with the inductor coilas at 6 and hence opens themain circuit only when danger impends.

. Switch 14 closes-the controlcircuit for the alarm signals 15 and 16. I have shown these to be a light and 4horn respectively, although they may beany positive device for calling attention tothe condition. -v i The effective insulation between coil and charge is the distance from the topturn of the inductor coil 1 to the charge 3 plus the distance from the bottom turn to the charge, hence by connecting mysignal circuit to the mid tap, I have in no way impaired the insulation value of the lining.

It will be noticed that if the charge leaks out to touch the bottom turn of the coil, there will be an electromotive force from the main lines '7, 71 through the solenoid 1l. As the frequency of the main source is generally of the order of 1,000 cycles, the reactance of the coil for the 60 cycle control circuit will be so great as to limitany current fiow from the main line to a safe figure. If the main supply frequency is low, then the solenoid 11 must be wound to stand the maximum current, and the control voltage must be raised to insure operation.

As the bath is grounded at all times, there is no chance to receive a current through the body when testing or charging the furnace.

In Figures 2 and 3, I have shown a shield 17 which may be inserted in the furnace lining to form one contact of the control circuit, instead of the coil. This shield is of any sheet metal, preferably non-magnetic and is cut as shown to prevent eddy current paths from being set up in the shield from the main inductor coil. The shield is bent to form a cylinder but the bottom connecting ring is not allowed to close on itself. In larger size furnaces where brick liners are used against the inductor coil, this shield can be suitably and easily inserted between two layers of brick. When metal leaks out of the charge to make contact with the shield, the signal circuit is closed as in the case where the coil formed the connection.

In low temperature work, the signal or control wires may be connected by pressure, Welding, or other means, directly to the charge. If the charge does not leak through in the molten state, it still offers protection againsta double electrical breakdown from coil to charge and back to coil, as the control circuit is completed through the arc.

In Fig. 4, I have shown a case somewhat similar to that just described.` Here the charge is of graphite and may be packed in lamp black or other heat insulating material, and insulated from the coil by mica or cement insulation, 18. If lamp black is used, it is'not necessary for the contact wire to touch the charge as the packing is itself conducting. I have shown this for conneotio 51, and in the same figure have indicated the double breakdown from coil, to charge, to coil, by the two arcs 61, 61.

If 'an arc is stopped, the instant it forms, there is little chance of its starting a water leak which will cause damage; but sometimes enough molten metal will follow a small leak to open up a big stream. In this case, steam is rarely trapped under the metal; but to add still further protection,

I have added a safeguarding feature which is energized simultaneously with the opening of the main circuit.

-In Fig. 6, I have shown a solenoid 20 in parallel with solenoid 11. This solenoid operates directly on armature 21 or through relays to close the water cooling inlet pipe 22, to the coil and open a compressed air inlet pipe 23. In this way, at'the first signal of distress, the water is not only disconnected from the inductor cooling system, but is blown out bythe compressed air.

In View 01' my invention and disclosure, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain part or all of the benefits of my invention without copying the structure shown, and Litherefore, claim all trical breakdown of the refractory lining betweenthe inductor coil and the charge. y

2. In an induction furnace having a refractory lining adapted tocontain a charge, an inductor coil surrounding said lining, leads connecting the ends of the inductor coil to a source of power supply, a signal circuit comprisingy 'a relay operated signal and an independent source of current supply in series and connected between an approximate mid tap on the inductor coil and the charge to indicate electrical breakdown of the refractory lining between the inductor coil and the charge and a separate signal circuit in conjunction with the first, operating on the same source of current supply as the first to indicatethe operating condition of the rst signal circuit and the contact that the first signal circuit makes with the charge.

3. In an induction furnace system an inductor coil, a refractory lining adapted to containl a charge. a signal system comprising a relay operated signal and an independent source of current supply in series and connected between said coil and charge, one branch of said signal system being connected to an approximate mid tap on the coil and the other being connected to the 8@ charge and to the ground. i

4. In an induction furnace system an inductor coil, an electrical and heat insulating lining therefor, adapted to contain a conducting charge, a source of alternating current connected to the ends of said inductor coil, a signal system comprising a relay operated signal and an independent source of electriccurrent connected between an approximatel mid point on the inductor coil and the conducting charge, a contactor in the main inductor coil line and relay means operating in conjunction with the signal device to open the main alternating current line and to operate the signal in event of an electrical breakdown between the inductor coil and the charge.

5. In' an induction furnace system an inductor coil, -a refractory lining adapted to contain a charge, a signal system comprisingva relay operated signal and an independent source of current supply in series and connected between said coil and charge, one branch of said independent source of supply being connected through the relay to an approximate mid tap on the coil and the other being connected to the charge and to the ground, and a secondary signal system operative- 1y connected between the ungrounded side of the source of current supply of the rst signal system and the charge to indicate that the main signal circuit is energized and that the contacts of said main signal circuit with the furnace charge are intact.

FRANK TI-IEODORE CHESNUT. 

